The present invention relates to a method of improving the exhaust-gas behavior of mixture-comprising internal combustion engines, in which the output signal of an oxygen measurement probe arranged in the exhaust gas channel of the internal combustion engine is fed to a controller the output voltage of which represents a setting variable for controlling the fuel-air ratio.
The invention furthermore relates to a device for the carrying out of the method.
In the art of preparing the fuel-air mixture for mixture-compressing internal combustion engines, it has been known for a long time to control the composition of the mixure as a function of the output signal of an oxygen measurement probe arranged in the stream of exhaust-gas in order to achieve the best possible elimination of noxious exhaust gases. In this case, the output signal of the oxygen measurement probe is fed to a controller, and the output voltage of the controller forms a setting variable for controlling the fuel-air ratio.
By the control of the fuel-air ratio of internal combustion engines, it is desired primarily to reduce noxious contents of the exhaust-gas emissions of internal combustion engines. For this purpose, a method is known, for instance, which employs an oxygen measurement probe arranged in the exhaust-gas stream of the internal combustion engine and controls an integrating device, the output signal of the exhaust-gas measurement probe being applied to a threshold switch, and switching the latter when the threshold valve is reached. The direction of integration of the integrating device is changed upon the switching of the threshold switch. The time constant of the integrating device is changed as a function of at least one operating parameter of the internal combustion engine, and particularly as a function of the amount of intake air of the internal combustion engine. The integrating device of variable time constant proposed in the case of the known method, however, does not satisfy all requirements with respect to precise and adaptable control. In particular, in the known method the setting variable does not follow sudden changes in loads sufficiently rapidly.
The voltage signal of an oxygen measurement probe typically has a steep course, in the region of lambda=1, which passes in each case into a flat branch directly adjoining this region. The voltage signal of the oxygen measurement probe acts on the means for forming the mixture in the manner that upon a signal from the probe which corresponds to a lambda of less than 1, the mixture is made leaner, while with a probe signal of more than 1, the mixture is made richer. As a result of the steep course of the characteristic curve, the control passes in quasi-steady state continuously through the region of the characteristic curve between the two points of inflection. On the average, the compostion of the mixture is such that it corresponds to the stoichiometric value and the exhaust gas can be substantially freed of undesired noxious portions by a catalytic exhaust-gas purification device.
Upon a sudden change in load, such as occurs frequently in the normal travel of an automotive vehicle, the value of the fuel-air ratio, lambda, can differ considerably from the desired value 1, primarily as a result of condensation processes in the intake region of the internal combustion engine. As a result of the flat course of the characteristic curve of the voltage value of the oxygen measurement probe with lambda values above or below a value of 1, however, only a relatively small disturbance signal is produced so that the control device only inadequately recognizes the actual deviation of the lambda value from the desired condition. Accordingly, the return of the control to the value of 1 takes an undesiredly long period of time and, during this time, the exhaust gases contain a high percentage of noxious substances. Furthermore, the continuous variation of the fuel-air ratio within relatively wide limits around a value of 1 results in a periodic change in the composition of the exhaust gas which must be counteracted in the following manner, namely, that both the exhaust-gas measurement probe and the catalytic after-treatment device for the purification of the exhaust gas are arranged at a minimum distance from the outlet of the internal combustion engine.
A method is already known in which a regulation in accordance with a stored typical field of characteristic curves is superimposed on the control of the fuel-air ratio. However, this method does not eliminate the difficulties inherent in the nonlinear course of the characteristic curve of the oxygen measurement probe. Furthermore, there is a considerable expense for memory in the electronic regulating device.